Grain-oriented electrical steel sheets are mainly used for iron cores in transformers or the like. In recent years, as demands for energy-saving iron cores have been increasing, the grain-oriented electrical steel sheets constituting the iron cores are required to have more excellent magnetic properties, i.e., low iron loss and high magnetic flux density.
A grain-oriented electrical steel sheet has a crystalline structure in which the <001> direction, which is the axis of easy magnetization of iron, is in excellent accord with the direction of rolling of the steel sheet. Such a texture is formed by secondary recrystallization, in which crystal grains with the {110} <001> direction, i.e., the Goss orientation, are preferentially grown into giant grains, during a finishing annealing (finish annealing) step in the process for producing the grain-oriented electrical steel sheet. Thus, the crystal orientation of the secondary recrystallized grains significantly affects magnetic properties of the grain-oriented electrical steel sheet.
Such a grain-oriented electrical steel sheet is produced by a known method described below. A steel slab containing about 4.5 percent by mass or less of Si and an element that forms an inhibitor, such as MnS, MnSe, AlN, or BN, is heated at 1,300° C. or higher, subjected to hot rolling, and, if necessary, subjected to hot rolled steel annealing (normalizing). The term “inhibitor” means fine precipitates that suppress an increase in the size of recrystallized grains resulting from primary recrystallization in order to achieve appropriate secondary recrystallization. In a step of performing secondary recrystallization, for example, the precipitates coarsen to weaken the pinning effect, resulting in the occurrence of the secondary recrystallization. Then, after hot rolling or hot rolled steel annealing, cold rolling is performed once, or twice or more times including intermediate annealing, to form a sheet having a final thickness. The resulting sheet is subjected to primary recrystallization annealing in a wet hydrogen atmosphere to perform primary recrystallization and decarburization. An annealing separator mainly composed of magnesia is applied to the sheet. To perform secondary recrystallization and purify the inhibitor-forming element, the sheet is subjected to finishing annealing at 1,200° C. for about 5 hours (for example, see U.S. Pat. No. 1,965,559, Japanese Examined Patent Application Publication Nos. 40-15644 and 51-13469).
However, such a method for producing the grain-oriented electrical steel sheet must includes heating the slab at a high temperature and performing finishing annealing at a high temperature and a long period of time, resulting in significantly high production costs.
In order to overcome the problems, the patent applicant corporation developed a method for promoting secondary recrystallization without an inhibitor (for example, Japanese Unexamined Patent Application Publication No. 2000-129356).
The method totally differs in technical idea from the known method for producing a grain-oriented electrical steel sheet. In the known method, secondary recrystallization occurs with a precipitate (inhibitor), such as MnS, MnSe, or AlN. In contrast, in the method for promoting secondary recrystallization without an inhibitor, the inhibitor is not used. A reduction in resistance to grain boundary migration due to purification actualize an intrinsic difference in the rate of grain boundary migration depending on the structure of high-energy boundaries (texture inhibition effect), thereby promoting secondary recrystallization. The method for promoting secondary recrystallization without an inhibitor does not require heating the slab at a high temperature or performing finishing annealing at a high temperature and a long period of time. That is, costs for purification of the inhibitor are not required; hence, it has been possible to produce the grain-oriented electrical steel sheet at low costs.